It’s impossible to answer this question without knowing:
1. Whether there are other offers or not
2. The location and it’s market (eg every London borough is showing different market behaviour)
3. How realistic the initial asking price is
4. Whether it’s leasehold or freehold
5. Whether the seller is in a chain or not
6. Whether its a typical or unusual property
7. How long ago it last sold, how much for I could go on.
Every offer situation is unique
Every offer made is influenced by a unique combination of:
1. The buyer’s financial situation and confidence
2. The seller’s motivation level
3. How well the EA has marketed the property
4. How many up front preparations have been made by each party to speed up the transaction.
For example, if you were a seller who needed to sell, and you’re expecting a typical 4+ month transaction time, how much less would you accept for someone who guaranteed to complete in 28 days? The answer is different to each seller, depending on their situation. But the point is, that time has a value in these transactions. Generally, the less time you have, the more it’s going to cost you, and vice versa.
That said…
All that said, if it’s a mainstream, typical property, if it’s been typically overpriced, if it’s been sitting on the market for a typical 3-6 months, if the seller needs to move, and if I really loved it, I’d be offering 20% below asking with immediate search pack purchase, a 28-day exchange target or less (subject to search pack results and lender survey). Most will refuse, but some will accept. You only need 1.